Vertical drilling into the earth, in an attempt to access oil and gas reserves, is a relatively straightforward operation. A drillstring comprising a drill bit at its end is rotated from the surface, such rotation powering the drill bit to cut into the earth. Typically so-called drilling muds are passed down the inside of the drill bit, which exit at the cutting face of the drill bit. These muds cool the drill bit, keep it lubricated and carry away the cuttings as they flow upwards in the annulus between the drillstring and the inside face of the drilled borehole.
However, if it is desired to change drilling direction, then so-called directional drilling is required, which is more technically challenging.
A common method of inducing a change in drilling direction is for the drillstring to include a slight bend of a few degrees near to the drill bit. By rotating the shaft, the drill bit can be “pointed” in the desired direction. Because it is desirable to maintain the drill bit pointed in a specified direction, a motor is provided to power the drill bit when the shaft is not rotating. The shaft is then held in position in a non-rotating manner, whilst the motor rotates the drill bit. Such a drilling mode is often called “sliding drilling” as the shaft of the drillstring effectively slides into the ground without rotating.
In this common arrangement, once the trajectory of the drill has deviated sufficiently, regular drilling in a straight line can be resumed by initiating rotation of the drillstring. This has the effect that the slight bend in the shaft begins to rotate and the net result is drilling in a straight line. Such a drilling mode is often called “rotary drilling”.
Thus, by moving between sliding drilling and rotary drilling modes the trajectory of drilling can be controlled.
However, in practice, this approach is fraught with difficulties.
Firstly, if the reactionary force on the drill bit changes significantly, as often happens, then this is transmitted, as “reactive torque” to the drillstring. This has the effect of the drillstring rotating near the drill bit, causing the drill bit to veer off from its target direction.
Secondly, as the lengths of the drillstring may be several kilometers, a substantial length will be in frictional contact with an inside face of the borehole. This makes correcting any deviation of the drill bit from its target direction by rotating the drillstring at the surface particularly difficult, as such rotations are often not transmitted to the drillstring at all and are instead stored in the shaft as rotational strain energy.
Such friction also makes it difficult to control the weight applied to the drill bit during sliding drilling. Additional weight applied to the drillstring at the surface can simply be absorbed by the drillstring as compressive strain energy, and in an extreme case providing no change to the weight applied to the drill bit.
This can result in an eventual release of stored compressive strain energy, which can result in a significant overshoot in the weight applied to the drill bit. This can damage the drill bit or cause it to stall, shortening the life of the bit and making the drilling operation take longer, both of which significantly increase the cost of drilling.
Methods of reducing the effect of friction during sliding drilling are known. For example U.S. Pat. No. 6,050,348 teaches “rocking” the drillstring at the surface to a specified angle to reduce the friction between the drillstring and an inner face of the borehole. Another method disclosed in U.S. Pat. No. 7,096,979 teaches “sliding” the drillstring by rotating the drillstring at the surface back and forth between specified torque limits. This is claimed to reduce wall friction during sliding drilling and therefore improve control of the direction of the drill bit and the weight applied to the bit.
However, even with these methods, significant deviations in drilling direction and lack of control of weight on bit are encountered during sliding drilling.
Moreover, downhole motors may be used in different types of drilling operations and the action of the downhole motor in rotating the bit/bottomhole assembly may cause a torque reaction from the operation of the downhole motor that may cause twisting of the drillstring and/or tool face instability.